The present invention relates to a recording signal generation system for a color video signal which is desirably applicable to high density recording in a magnetic recording and playback apparatus.
Various magnetic recording and playback apparatuses have been proposed in the art of video tape recorders (VTR) and like magnetic recording and playback apparatuses for the purpose of recording information signals in high densities.
For example, in a magnetic recording and playback apparatus of the type using a magnetic recording medium, particularly a magnetic tape, use is made of two read and write heads each having a head gap whose lengthwise position is inclined a small angle (.+-.6-7 degrees) relative to a direction perpendicular to an extension of a track on a magnetic tape, i.e., two read and write heads having head gaps which are different from each other in azimuth angle. The heads undesirably trace adjacent tracks on the magnetic tape but minimize reproduction of information in the adjacent tracks, or crosstalk, causing a so-called azimuth loss which is derived from the magnetic recording and playback theory. Such eliminates the need for a guard band heretofore provided between adjacent tracks, and thereby allows information to be written into the magnetic tape in a high density.
In a recording and playback system of the above-described type which uses the azimuth effect, information in adjacent tracks is minimized to be reproduced due to the azimuth loss so long as recorded signals have relatively short wavelengths, that is, they belong to a high frequency band. This eliminates a beat problem on the reproduced pictures which would otherwise be caused by signals in a high frequency band recorded in adjacent tracks. However, where the recorded signals have relatively long wavelengths, that is, they belong to a low frequency band in which the azimuth effect is insufficient, signals in adjacent tracks would appear in a reproduced signal as crosstalk. Assume, for example, a case wherein the signal to be magnetically recorded and reproduced is a signal generated by multiplexing a frequency modulated version of a luminance signal, which occupies a high frequency band, and a version of a carrier color signal which has been converted into a frequency lower than the frequency modulation (FM) band of the luminance signal, i.e. low frequency band converted carrier color signal. In such a case, no azimuth effect is expected of the low frequency band converted carrier color signal, resulting in incomplete removal of the crosstalk.
To solve the problem discussed above, there have been proposed a magnetic recording and playback apparatus employing the so-called "carrier offset system" in which the frequency of the carrier color signal is selected to interlace successive tracks, as disclosed in Japanese Patent Laid-Open Publication No. 50-34419/1975, a magnetic recording and playback apparatus employing the so-called "PI system" in which the phase of a carrier color signal in one track is changed by 180 degrees for each horizontal scan period, 1H, as disclosed in Japanese Patent Laid-Open Publication No. 50-42733/1975, a magnetic recording and playback apparatus employing the so-called "PS (phase shift) system" in which the phase of the carrier color signal is rotated 90 degrees for each 1H and the rotating direction is changed track by track, as disclosed in Japanese Patent Laid-Open Publication No. 52-48919/1977, etc. Such implementations have won worldwide popularity as well known in the art.
The problem heretofore pointed out is that the various systems mentioned above cannot effect desirable recording or reproduction of a color video signal unless the color video signal conforms to a color TV system in which a luminance signal and a carrier color signal are multiplexed sharing the frequency band of the luminance signal, e.g. NTSC (National Television System Committee) System or PAL (Phase Alternation Line) system. In the SECAM (Sequentiel a Memoire) system, for example, a color video signal is made up of a luminance signal and color signals which are in an FM signal mode and, therefore, the FM wave of the color signals, even if recorded and reproduced after being shifted to a lower frequency range, do not have horizontal correlation. It follows that even the various systems previously described fail to fully eliminate crosstalk from adjacent tracks. Except for the problems discussed later in this specification which led the present invention, such a problem may be solved by recording a color video signal of which a luminance signal and color signals are time-division multiplexed.
Today, the time axis of a signal is readily adjustable using semiconductor devices. This has promoted transmission and recording of a color video signal having a specific mode in which a luminance signal and time-axis compressed color signals are arranged serially on the time axis. As well known in the art, in the NTSC color TV system, a color video signal is allowed to occupy a narrow frequency band taking advantage of a peculiarity of human eyesight which is far poorer for a difference in color in a small area on a screen than for a difference in brightness in such an area. For example, concerning the Japanese standard color TV system (common to the NTSC system), while the luminance signal representing brightness information occupies a frequency range of 0-4 MHz, the color signals representing color information are 1.5 MHz for a wide band signal (I signal) and 0.5 MHz for a narrow band signal (Q signal). Also, the luminance signal and the color signals are kept in a mode which allows them to be separated from each other within an effective horizontal scan period in one horizontal scan period, 1H, which accords to predetermined scanning standards.
In one horizontal scan period, 1H, the effective horizontal scan period for which the luminance signal and the color signal coexist and a horizontal blanking period which is shorter in duration than the effective horizontal scan period and has a horizontal sync signal therein are arranged serially on the time axis. For this reason, time-division multiplexing of the luminance signal and the color signals is attainable if one of them which separably coexist in an effective horizontal scan period is put in the effective horizontal scan period, and the other in a blanking period after time-axis compression. In time-division multiplexing a luminance signal and color signals, it is desirable from efficient band utilization and other viewpoint that the color signals which occupies a narrower frequency band than the luminance signal does, be time-axis compressed to become color signals whose occupying frequency band is substantially the same as that of the luminance signal, and be arranged in a horizontal blanking period. The two color signals are time-division multiplexed with the luminance signal alternately at every interval of one horizontal scan period, the multiplexed signal being a so-called time-sequential signal.
In the color video signal having the above-described mode, the luminance signal and the color signals have been time-division multiplexed and, therefore, they will never exist at the same time or interfere with each other. Concerning energy distribution, both the luminance signal and the color signals are large in the low frequency range and small in the high frequency range and such is suitable for frequency modulation. Furthermore, the color video signal of the type concerned allows a minimum of time-axis fluctuation to occur during a recording or playback operation with a recording medium, compared to the NTSC system or the PAL system which employs a color subcarrier. Additionally, in the system using two heads having azimuth angles, no anti-crosstalk measure is required because the color signal will never appear in the low frequency range which often entails crosstalk.
FIG. 1 shows an exemplary waveform of a color video signal having a luminance signal arranged in an effective horizontal scan period and time-axis compressed color signals in a horizontal blanking period, as previously described. The illustrated color video signal is generated by time-division multiplexing a luminance signal and compressed color signals under a condition wherein the effective horizontal scan period of 52 microseconds of the CCIR (International Radio Consultive Committee) signal having 625 scan lines an interlace scan rate of 2, and 25 pictures per second (50 fields per second) has been modified to 50 microseconds.
In FIG. 1, the duration of 64 microseconds is the one horizontal scan period and that of 50 microseconds is the effective horizontal scan period in which a luminance signal is disposed. The duration of 14 microseconds is the horizontal blanking period in which, for the period of 4 microseconds, a horizontal scan signal and a DC level for color reference are arranged and, for the period of 10 microseconds, compressed color signals are arranged. The color signals to be multiplexed with the luminance signal are shown as signals (R-Y)c and (B-Y)c which are the compressed versions of color difference signals (R-Y) and (B-Y) respectively.
The color video signal shown in FIG. 1 is one which has been proposed as being applicable to a magnetic recording and playback apparatus.
As described above, in the color video signal of FIG. 1, a 4-microsecond period is employed to arrange a horizontal sync signal and a DC level for color reference within the horizontal blanking period, while color signals having a duration of 50 microseconds are compressed to be the signals (R-Y)c and (B-Y)c having a duration of 10 microseconds. Therefore, the effective horizontal scan period is 50 microseconds which is the difference between one horizontal scan period, 64 microseconds, and the duration of (10+4) microseconds.
A problem encountered with the proposed color video signal is that the effective horizontal scan period is 50 microseconds as already mentioned which is 2 microseconds shorter than the original effective horizontal scan period in the CCIR signal, 52 microseconds, the information signal being omitted for the duration of 2 microseconds. While this problem may be solved by time-axis compressing the luminance signal as well as the color signals, difficulty is experienced in so compressing the luminance signal because it is a wide band signal and the apparatus becomes costly and intricate.
Further, although such a color video signal includes two different kinds of color signals which are line sequentially arranged on the time axis, it does not contain any color signal discrimination data (color discriminating signal) on a horizontal scan period basis. Therefore, any disturbance caused in the signal condition for one reason or another would render the signal processing inadequate. This also holds true for the SECAM system, for example, which contains color signal discrimination data in a vertical blanking period. The term "time compressed" is commonly used in the art in systems to which the present invention pertains and may be found, for example, in U.S. Pat. No. 4,245,235.